Abstract : Substances produced by organisms living and growing in water are of special interest to naval architects, since their presence may greatly change frictional resistance determinations made in the towing tank and at sea. To indicate the possible changes in resistance that might be encountered, experiments with fresh-water and marine plankton cultures, as well as with larger algae, are described. In these experiments, friction reductions of as much as 65% were recorded. The influence of these high-polymer substances on cavitation inception, appearance, and damage is now under active study. Preliminary findings regarding their effects are presented. Methods of detecting the presence of algae-produced compounds are also discussed, together with their possible effects on towing-tank, water-tunnel, and at-sea testing techniques.